1. Field of the Invention
The present invention relates to an inkjet recording apparatus and an inkjet recording method, and more particularly to technology for improving image formation quality in an inkjet recording apparatus having a shuttle scanning system which forms an image by performing high-frequency ejection from an inkjet head while reciprocally moving a carriage on which the inkjet head is mounted.
2. Description of the Related Art
In the shuttle scanning inkjet system, it is often that signals specifying droplet ejection timings of an inkjet head (referred to as “ejection trigger signals”, “ejection timing signals”, “ejection clock signals”, etc.) are generated in accordance with positional signals indicating the position of the inkjet head which is obtained from an optical linear encoder arranged along the movement direction of a carriage (main scanning direction) on which the inkjet head is mounted (see Japanese Patent Application Publication Nos. 2009-034839 and 2004-299348, for example).
In general, the linear encoder is constituted of a transparent sheet (scale) in which a pattern of black stripes having a line density in a range of approximately 150 lines per inch (lpi) to 300 lpi is formed, and a light emitting part (including a light-emitting diode (LED), for example) and a light receiving part which are arranged to face each other across the transparent sheet, to detect the shades of the stripes to output signals to be used as the positional signals. It is also possible to arrange two light receiving parts at an interval that is equal to ¼ of the line pitch of the scale so as to obtain two sinusoidal output signals having phases shifted from each other by 90°, which are used to generate the ejection trigger signals achieving the recording resolution that is four times high as the line density in the scale. For example, when the scale in the linear encoder has the line density of 150 lpi, it is possible to generate the ejection trigger signals corresponding to the recording resolution of 600 dots per inch (dpi), by means of the composition in which the two light receiving parts are arranged at positions separated by ¼ of the line pitch of the scale.
If it is necessary to obtain the ejection trigger signals for even higher resolution, then the ejection trigger signals for 1200 dpi, 2400 dpi, or the like, can be generated from the timing signals corresponding to 600 dpi by using a multiplier. The multiplier can include a phase locked loop (PLL) circuit, or can calculate approximate timings by calculation using high-frequency clock signals (see Japanese Patent Application Publication No. 2009-214326, for example).